The present invention relates to the medical diagnostic and surgical arts. It finds particular application in conjunction with neurosurgery and will be described with particular respect thereto. It will be appreciated, however, that the invention finds application in conjunction with biopsies, endoscopic procedures, orthopedic surgeries, other medical procedures, industrial quality control procedures, and the like in which a tool or device must be accurately positioned in relation to an object.
Image guided surgery systems are particularly well adapted to intra-cranial and spinal surgery. These systems use diagnostic images of the patient to assist the physician with presurgical planning and to provide information relating to the position and orientation of the anatomy and instrumentation during a surgical procedure. Image guided surgery systems are well-suited for use in connection with magnetic resonance ("MR") and computerized tomography ("CT") images, as well as with other imaging modalities.
In cranial applications, a patient reference frame may defined using three or more points fixed in relation to the patient's head. According to one method, at least three markers visible to the imaging device are affixed to the skin prior to imaging. According to another method, anatomical reference points are used. According to a third method, fiducial markers may be affixed to the skull, for example as disclosed in U.S. Pat. No. 4,991,579, Method and Apparatus for Providing Related Images of the Anatomy over time of a Portion of the Anatomy Using Fiducial Implants, to Allen, issued Feb. 12, 1991. Similar techniques may be used to define a patient reference frame with respect to other portions of the anatomy.
An image of the patient having an image reference frame is then obtained. Based on the location of the three or more markers within the image data, the image and patient reference frames can be correlated. Hence, the position of a feature of interest within the image can be determined with respect to the patient reference frame. After image acquisition is complete, the patient can be moved as desired. The patient is subsequently placed in an operating room environment, for example on an operating table.
A localizer, which defines an operating room reference frame, is used to determine the position of a surgical tool. The tool includes a plurality of emitters having a known relationship to the tip of the tool. Based on the signals detected by the cameras, the position of the tool with respect to the operating room reference frame may be determined.
The patient and operating room reference frames are correlated or "zeroed" by touching the surgical tool to the at least three markers. The position of the tool with respect to the cameras, and hence the position of the markers, is then determined. Inasmuch as the relationship between the patient, operating room, and image frames of reference is known, the position of the tool with respect to the image reference frame can then be determined. Relevant images, with the position of the surgical tool indicated thereon, are then displayed on a monitor. The surgeon is thus provided with a real time indication of the position of the surgical tool with respect to the previously obtained image.
In spinal applications, the patient reference frame is preferably defined in relation to anatomical reference points such as the transverse and spinous processes. The patient and operating room reference frames are correlated by touching the surgical tool to the reference points.
The position of the patient ordinarily remains fixed with respect to the localizer. Should the position of the patient change, however, the patient and operating room frames of reference must be re-correlated by touching the tip of the probe to the at least three markers.
As an alternative, the position of the patient may be continuously monitored through the use of one or more emitters having a known position with respect to the patient. By determining the position of the emitters with respect to the localizer, the patient and operating room reference frames may be automatically re-correlated to account for movement of the patient.
U.S. Pat. No. 5,383,454 (the "'454 Patent") to Bucholz, entitled System for Indicating the Position of a Surgical Probe Within a Head on an Image of the Head, discloses an apparatus usable with a BRW head ring. A reference ring is attached to the head using sharp pins. Prior to imaging, a cylindrical reference frame is attached to the reference ring. Image data is obtained with the head encircled by the frame. By determining the position of the reference frame in the image data, the relationship between the data and the reference ring may be determined. Prior to surgery, the reference frame is removed from the reference ring and replaced with a base ring containing a plurality of emitters. The position of the emitters is then monitored by localizer, and the operating room and patient reference frames are automatically re-correlated to account for movement of the patient.
A drawback to the apparatus disclosed in the '454 Patent is that the reference ring and frame are bulky, obtrusive, and uncomfortable. As a result, the reference ring and frame are generally worn for only a short period of time. Further, the reference ring complicates the surgeon's access to the patient's anatomy. Yet another drawback is that an additional clamping device is needed to immobilize the patient's head during surgery. Again, however, the structure of the reference ring requires that the head clamp be designed and positioned to avoid mechanical interference with the reference ring.
The '454 Patent also notes that the reference ring and frame may be eliminated by using surgical screws affixed to the patient's skull to identify reference points on the patient's body and suggests that the emitters may each be separately mounted to a screw or other fixed structure positioned at one of the reference points.
A drawback to this approach is that it complicates the mechanical structure of the screws or pins attached to the reference points. Not only must the screws be visible in the scanned image and facilitate accurate correlation of the various reference frames, the screws must also accommodate attachment of the emitter structure. At the same time, it is desirable that the screws be as unobtrusive as possible. Further, the screw or screws must be positioned so that the emitter structure does not interfere with the head clamp and so that the emitters communicate reliably with the localizer.
U.S. Pat. No. 5,269,304 (the "'034 Patent") to Day, et al., entitled Surgical Head Clamp discloses a head clamp which is marketed under the Mayfield.RTM. trademark by Ohio Medical Instrument Company, Inc. of Cincinnati, Ohio. With particular reference to FIG. 2 and column 2 line 36 to column 3 line 26 of the '034 Patent, expressly incorporated by reference herein, the head clamp includes an adjustable, generally c-shaped frame (reference numeral 12 in the '034 Patent). The clamp also includes three head engaging pins, two of which are mounted on a rocker arm (reference numeral 28 in the '034 Patent), which is in turn adjustably mounted to the c-shaped frame 12. The clamp is in turn attached to a structure such as the operating table.
Reference emitters have been mounted on a structure clamped to the operating table. When mounted to the operating table, however, the emitters are useful for providing information as to movement of the table but fail to provide information as to movement of the patient with respect to the table. Reference emitters have also been mounted to the c-shaped frame 12 of the clamp described in the '034 Patent and thus provide tracking of the clamp position together with an indication of the position of the patient's head. However, the clamp includes various adjustments which facilitate positioning of the clamp and the patient, as well as various mechanical connections between adjacent parts. Each of these adjustments and connections has the potential to allow relative motion between the emitters and the patient such that movement of the patient may not be accurately reflected in the position of the c-shaped frame 12, or vice versa.